DFT insights into the stability of single-metal-atoms on Mo-based o-MXenes driven by the ligand effect

MXenes, a novel class of two-dimensional transition metal carbides, nitrides or carbonitrides, have emerged as promising supports for single-atom catalysts owing to their tunable structural and electronic properties. Using density functional theory calculations, we investigate ordered Mo-based bimetallic MXenes without surface functional groups ( and o-Mo₂M″C₂, where M″ = Ti, Zr, Hf, V, Nb and Ta) as materials to stabilize platinum-group-metal (PGM) single atoms at Mo vacancies. Compared with the corresponding Mo-only MXenes, introducing a second metal component, M″, strengthens PGM anchoring and increases sintering resistance, which we attributed to a ligand effect that enhances PGM-C covalency and reorganizes the PGM d states. This stabilization is further captured by a combined electronic descriptor δ = ε_p - ε_d. Here, δ is defined as the difference between the p band center (ε_p) of the C atoms adjacent to the PGM and the d band center (ε_d) of the PGM atom. This descriptor δ shows a strong correlation with the binding strength between PGM and MXenes. To assess the changes of PGM reactivity raised by different MXenes, we use CO, CH₃ and NH₃ adsorption strength, highlighting a stability-reactivity trend. An XGBoost-based machine learning analysis further revealed that adsorption energies are governed by multiple electronic descriptors (beyond ε_d), with the average electronegativity (χ_avg) of the PGM-MXene system contributing to the observed trends in adsorption energies. This study provides a new atomic-level understanding of the rational design principle for stabilizing atomically dispersed SACs on MXenes and provides guidelines for the experimental synthesis.